I don't believe Firefox is able to distinguish between an external and internal microphone, hence why the permission is simply labelled microphone.

You might be able to restrict Firefox permission for a specific microphone device from without your operating system settings.

Hope this helps.

The drop-down box lists it as the "external microphone" which could be a text error. The operating system would allow the browser to know which microphone, external vs internal, is in use

Are you able to attach a screenshot of where Firefox gives the option between external and internal microphone? I don't see that option on my browser.